The present invention relates to a technique for a service in which information related to road traffic is provided, and particularly to a technique useful for being applied to communication equipment (or a communication device), an inter-vehicle communication control method, and an inter-vehicle communication system by which information for indicating vehicle running conditions is transmitted and received.
In recent years, an intelligent transport system (ITS) has been widely spread for the purpose of safe-driving support for vehicles in road traffic and efficiency of road traffic. As one of services for supporting safe driving of vehicles in the ITS, there has been studied a service in which a vehicle having wireless communication equipment mounted receives traffic information such as position information of peripheral vehicles and traffic conditions from a roadside communication station installed on the side of a road or another vehicle having the wireless communication equipment mounted, and a warning is given to drivers to avoid a collision accident between vehicles. For example, the service is described as follows. The wireless communication equipment mounted in a vehicle periodically measures vehicle running information such as the position, moving direction and velocity of the host vehicle, and periodically transmits a vehicle running information packet. Then, another vehicle that received the vehicle running information packet calculates the possibility of collision using the received vehicle running information. In the case where it is determined on the basis of the calculation result that the possibility of collision is high, drivers are alerted by displaying a warning to urge them to stop the vehicles. However, such a service can be provided without a problem in a state where the wireless communication equipment of the source vehicle can directly communicate with that of the destination vehicle. However, in a state where vehicles are apart from each other, direct communications cannot be established and information cannot be transmitted in some cases. In such a case, a multihop communication system is employed so that the vehicle running information is transmitted to a destination while being relayed and transferred by vehicles and roadside base stations located between the source vehicle and the destination vehicle. As an example, Patent Documents 1 and 2 described later disclose a communication method using the multihop communication method.
The method disclosed in the Patent Document 1 is a method in which when information is transmitted to peripheral vehicles by broadcasting, peripheral vehicles located apart from the host vehicle are determined as relay vehicles on branched roads, and the transmission information is transferred by the relay vehicles to reduce the traffic volume and to transmit information to wide areas.
The method disclosed in the Patent Document 2 is a method in which the density of peripheral vehicles is calculated on the basis of the relative position obtained using position data including the positions, running velocities, and running directions of a transmission vehicle and a reception vehicle, and a time required for periodically transmitting vehicle running information is set in accordance with the density of vehicles to reduce the number of packets of the vehicle running information.